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Also found as an A/C compressor adjuster on early 80s Cadillac engines.
It's really nothing but a LONG 4", full-thread, bolt.
I BELIEVE it's fine-threaded 5/16"-24 IIRC. At least grade 5. But I haven't messed with one for a long time. It could even be a 5/16-18 coarse thread. I just don't recall. Thread-check your old bolt or the adjuster portion where the bolt goes through to determine actual thread size you need just to make sure. You may be able to find a suitable bolt locally for a lot less.
Bolt only has one piece. The threaded adjuster goes to the alternator ear, and the bolt threads through the adjuster. The stop is what the bolt rests on.
Bolt (12), adjuster (13), and the stop (20) in the pic below is what all you need.